1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) likely to be interrogated, contactless and wireless, by a unit (generally fixed), called a read and/or write terminal. The present invention more specifically relates to read/write terminals of transponders with no autonomous power supply, which extract the power supply required by the electronic circuits included therein from a high-frequency field radiated by an antenna of the terminal.
2. Discussion of the Related Art
FIG. 1 very schematically shows in the form of blocks an electromagnetic transponder read terminal 1 and a transponder 10 intended to communicate with such a terminal.
On the read terminal side, a series oscillating circuit 2 formed of an inductance L1, forming an antenna, in series with a capacitor C1 (and generally a resistor R1), between an output terminal 3 of an antenna coupler 4 and a reference terminal 5, generally the ground, can be found. Coupler 4 receives from an amplifier 6 (PA) an excitation signal Tx provided by a modulator (not shown) of signals to be transmitted to transponder 10. The modulator belongs to circuits 7 (CIR) schematically shown in the form of a block comprising circuits for controlling the oscillating circuit and for exploiting data received from the transponder (among others, a modulator and a microprocessor for processing control and data signals). In the absence of data to be transmitted, the modulator of circuit 7 transmits a remote-supply carrier (generally at 13.56 MHz) directed to transponder 10. The junction point of capacitor C1 and of the inductance forms, in the example shown in FIG. 1, a terminal for sampling a data signal Rx, received from a transponder 10, and intended for a demodulator 8 (DEM). An output of demodulator 8 communicates the data received from the transponder to digital circuits 7 (generally, the microprocessor via a decoder).
To adapt the transmit power of the terminal, coupler 4 is generally used to sample an information proportional to the signal provided by amplifier 6 intended for a comparator 9 (COMP) controlling the gain of amplifier 6. This comparator compares, for example, the voltage sampled from a so-called CPLD terminal of coupler 4 with respect to a predetermined reference voltage Vref. In known fashion, a coupler 4, be it with coupled lines or local elements, comprises two terminals IN and DIR between which flows the main signal, and a terminal CPLD providing a proportional information. Generally, a fourth terminal ISO is left in the air or connected, by a resistor or a capacitor, to ground.
On the side of transponder 10, an inductance L2 in parallel with a capacitor C2 forms a parallel oscillating circuit (called a resonant circuit) intended to sense the magnetic field generated by the series oscillating circuit of terminal 1. Terminals 11 and 12 of the resonant circuit are connected to two A.C. input terminals of a (halfwave or fullwave) rectifying bridge 13 having their rectified output terminals 14 and 15 providing a supply voltage to circuits 16 (IC) of the transponder, generally via a capacitor, not shown, intended to store and smooth the voltage rectified by bridge 13. Electronic circuits 16 of the transponder generally essentially include a microcontroller and a demodulator of the signals possibly received from terminal 11. Circuit 16 receives a signal directly sampled across the oscillating circuit (for example, on terminal 11) to restore a clock signal from the remote-supply carrier provided by the terminal. Most often, all the electronic circuits of transponder 10 are integrated in a same chip, itself inserted in a smart card.
The transmission of data from transponder 10 to terminal 1, is performed under control of a stage of modulation (back modulation) of resonant circuit L2-C2. This modulation stage is generally formed of an electronic switch 17 (for example, a MOS transistor) and of a resistor R (or of a capacitor), in series between terminals 14 and 15 (or between terminals 11 and 12). Switch 17 is controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller than the frequency of the excitation signal of the oscillating circuit of terminal 1. When switch 17 is on, the transponder's oscillating circuit is submitted to an additional damping with respect to the load formed by circuit 16, so that the transponder draws a more significant amount of power from the high-frequency magnetic field. On the side of terminal 1, amplifier 6 maintains the amplitude of the high-frequency excitation signal constant due to the control performed by coupler 4 and comparator 9. Accordingly, the power variation of the transponder translates as an amplitude and current phase variation in antenna L1. This variation is detected by demodulator 8 of terminal 1, generally an amplitude demodulator.
The resonant circuit (L2-C2) of transponder 10 and the oscillating circuit (R1-L1-C1) of terminal 1 are generally tuned to a same frequency which most often corresponds to the remote-supply carrier frequency.
The impedance of the oscillating circuit of terminal 1 is generally adapted to the output of coupler 4, said coupler further having an input impedance adapted to the output of amplifier 6. For the impedance of the oscillating circuit to be adapted to the output of coupler 4, the imaginary parts of the respective impedances of inductance L1 and of capacitor C1 must mutually cancel at the tuning frequency, and resistance R1 (plus the series resistance of inductance L1) must have the value of the output impedance of the coupler (generally, 50 ohms).
A problem of conventional read/write terminals is that in case of a mismatch of the impedance of the oscillating circuit's antenna, the signal received by the demodulator is disturbed.
Now, any element entering the terminal's magnetic field is likely to create an additional reactive element in this field and to then modify the impedance of the oscillating circuit.
Further, the tuning of the resonance frequency to the carrier frequency is performed manually by means of a variable capacitor C1, once the terminal has been manufactured. This need for adjustment of capacitor C1 is especially due to the manufacturing tolerances of the capacitive and inductive elements. Generally, capacitor C1 has a capacitance value tolerance on the order of 20% and antenna L1 is manufactured with a tolerance on the order of 10%. Such tolerances are incompatible with the fulfilling of tuning accuracy constraints. Similarly, such tolerances adversely affect the impedance matching between amplifier 6 and the oscillating circuit.
A manual adjustment of a capacitor C1 requires a maintenance operation once the terminal has been arranged in its definitive environment. Further, maintenance problems linked to the impedance matching drift according to environment modifications, for example, after temperature and/or humidity changes, can also be observed.